KR101360535B1 - High strength thin cold-rolled sheet via cold rolling process and method for manufactureing the cold-rolled sheet - Google Patents

Abstract

The present invention relates to a cold rolled steel sheet used in a product requiring high strength and high formability, such as for home appliances or structural use, having a yield strength of 650 MPa or more and a hardness of 400 or more based on a Vickers hardness of 500 g, when r = 0 bending molding The present invention relates to a high strength ultra-thin cold rolled steel sheet using cold rolling having excellent moldability in which no crack is found in a bending part of a product, and a method of manufacturing the same.
The method for producing a high strength ultra-thin cold rolled steel sheet using cold rolling of the present invention is weight%, carbon (C): 0.15 to 0.25%, manganese (Mn): 1.5 to 2.5%, silicon (Si): 0.1 to 1.0%, titanium (Ti): 0.01 ~ 0.05%, boron (B): 5 ~ 30ppm, hot rolling step of hot finishing rolling a steel slab containing the balance Fe and other elements at an Ar ₃ temperature of 950 ℃ or less, the rolled steel sheet Winding step of winding at 500-800 ° C., cold rolling step of cold rolling the wound steel plate at 50 ~ 90% rolling rate, cold rolled steel plate in continuous annealing line at an annealing temperature of 750-850 ° C. for more than 30 seconds After the holding, cooling to a temperature range of 250 to 450 ° C., holding at this temperature for 50 seconds or more, and then cooling the annealing step and the second rolling of the annealing steel sheet treated as above at a rolling reduction of 10% or less. Include.

Description

High-strength ultra-thin cold rolled sheet using cold rolling and its manufacturing method {HIGH STRENGTH THIN COLD-ROLLED SHEET VIA COLD ROLLING PROCESS AND METHOD FOR MANUFACTUREING THE COLD-ROLLED SHEET}

The present invention relates to a cold rolled steel sheet used in notebooks, LCD monitors and products requiring high strength and high formability, such as parts for supporting strength of chassis such as LCDs, PMPs, and LED TVs. Yield strength over 650MPa and Vickers hardness up to 400g based on 500g hardness, r = 0 Bending high strength using cold rolling with excellent formability that no cracks are found in the bending part of the product An ultra-thin cold rolled steel sheet and a method for manufacturing the same.

In the case of a cold rolled steel sheet, which is mainly used in the conventional automobile body, electrical appliances, consumer electronics, especially durable consumer materials, low carbon steel series is mainly used for formability, the strength aspect tended to be relatively unconsidered. In particular, in the case of steel grades of EDDQ or higher requiring high formability, the strength was not increased to a specific value by focusing on the formability.

However, in recent years, low cost, high fuel consumption, slim (Slim), etc. is required, while maintaining the formability as previously, a cold rolled steel sheet having a thinner and higher strength characteristics is required. That is, by using ultra-high strength cold rolled steel sheet, it is possible to realize low cost by reducing the total weight of the steel used in the product, and high fuel efficiency is possible by reducing the total weight of products such as automobiles, and to make thinner products. Since the design of the product can also be diversified, it is possible to meet the requirements of low cost, high fuel efficiency, slimming, and the like.

Thus, in recent years, many studies have been conducted for the development of ultrathin products having high strength and high formability.

These studies are largely related to: 1) reinforcement of the structure (transformation) using transformations occurring during the steel sheet manufacturing process, 2) solid solution strengthening to control components that can be employed in steel, and 3) precipitation strengthening to increase the strength by distributing precipitates, 4) Finally, the steel sheet, which has been completely recrystallized through annealing, may be further rolled to work hardening, which causes work hardening.

If the prior art is largely classified into two categories, the process can be divided into 1) DR (Double Reducing) type process using secondary rolling and 2) DR abbreviated process not using secondary rolling. have. In other words, the transformation, solid-solution strengthening, precipitation strengthening, and the like may be classified into the DR process type and the DR omitted process according to the presence or absence of secondary rolling.

Among them, in the case of the DR process-type process in which the strength is increased by using secondary rolling, defects such as dislocations in the steel are inevitably generated due to the increase in strength due to the secondary rolling. While gradually increasing, the elongation is drastically reduced, so it is difficult to use it in an extremely hard part.

For example, steel sheets using secondary rolling have most elongation levels of less than 2 to 3% and crack in the rolling direction due to the deterioration of formability due to the low elongation and the effect of rolling grains generated during secondary rolling. This situation is a form of vulnerability.

Dividing these prior arts into carbon contents in steel, it is generally ultra-low carbon steel having a carbon content of less than 0.01 wt%, low carbon steel with a carbon content of 0.01 <wt% C <0.1, 0.1 <wt% C <0.25 It can be divided into carbon-based bicarbonate steel, and high carbon steel having a carbon content of 0.25wt% or more.

Looking at the prior art, the ultra low carbon steel is mainly used as a steel sheet for cans, and the conventional technique for this is to reduce the reduction ratio of the secondary reduction and improve the strength by controlling the content of Mn (JP1995-274558) and its The improved patent (JP1997-216980) which adjusts a reduction ratio for workability improvement, etc. are mentioned.

Moreover, the patent (JP2002-307898, JP2002-201574) etc. which improve the high temperature strength by using solid solution strengthening and precipitation strengthening, such as Mn, P, TiC, etc. are also proposed. However, in the case of ultra-low carbon steel, there is a limit in the strength and the elongation falls to a very low level during the secondary rolling to improve the strength, there is a problem in producing high formability and high strength products.

In addition, most of the high-strength steel sheet of low carbon steel is used as a black plate (BP) for cans, and the conventional technique for this is using high nitrogen steel and low pressure reduction of DRM (Double Reducing Mill) using DRM low pressure. (JP1990-052642), technology of increasing the content of Mn, continuous lubrication rolling, secondary rolling (JP1996-239734), technology using the effect of overaging treatment (JP1997-040883), technology using rapid cooling (JP2006-074140) etc. are mentioned.

However, even in these prior arts, the low carbon steel has a low strength level, and even if the strength level is high, it requires a high cooling rate that is difficult to realize in a general continuous annealing process, or the final elongation range obtained is lower than a target range. There is a limit.

In addition, in the case of high carbon steel of more than 0.2wt%, most of the high strength of the initial stage is not only difficult to roll down in PCM, but also difficult to leveling for shape control after rolling, which is not applied to ultra-thin cold rolled materials.

In recent years, these concepts have been combined to solidify and strengthen the matrix structure using P in a heavy carbon steel sheet. At the same time, the matrix structure is made into a two-phase structure of ferrite + pearlite, and the secondary rolling is controlled to 10% or less. Steel plates have been developed to maximize the combination of strength and elongation (KR2009-0084530).

In particular, this patent utilizes all of the above-described solid solution strengthening, structure control, and work hardening using a secondary rolling process, and the strength level is higher than that of other technologies (YS> 650 MPa). A method of providing an ultra-thin cold rolled steel sheet having excellent properties is provided.

However, these patents have a problem that the process is complicated by using secondary rolling, and although the rolling amount is small, dislocations are generated by the effect of rolling, resulting in a difference in formability in the rolling direction and the rolling vertical direction.

The present invention is to solve the problems of the prior art, by controlling the emphasis and the manufacturing conditions appropriately and very small secondary rolling less than 10%, high strength ultra-thin cold rolled steel sheet without the phenomenon of deterioration of the rolling direction formability And to provide a method for producing the same, there is a purpose. In particular, by adding elements such as Mn, B, which is a hardenability element, the hardenability is improved to high strength, and annealing is performed again, followed by secondary rolling, to form a super-thin cold rolled steel sheet having a high strength of 1000 MPa grade and a method of manufacturing the same. To provide.

As a means for realizing this cold rolled steel sheet according to the present invention,

By weight%, carbon (C): 0.15 to 0.25%, manganese (Mn): 1.5 to 2.5%, silicon (Si): 0.1 to 1.0%, titanium (Ti): 0.01 to 0.05%, boron (B): 5 30 ppm, residual Fe and other unavoidable elements, and the structure includes bainite single phase or 70 vol.% Or more of bainite and residual ferrite.

Further, it is preferable that the product of the C, Mn and B contents satisfies the relationship of 1.13 x 10 ^-4 <wt% C x wt% Mn x wt% B <1.875 x 10 ^-3 .

In addition, the thickness of the cold rolled steel sheet is preferably 0.5mm or less.

In the cold rolled steel sheet, when the r = 0 L-bending molding test is performed, the number of cracks that can be visually observed at the corners thereof is preferably 2 or less per unit m.

In addition, the method for producing a cold rolled steel sheet according to the present invention as a means for realizing this,

By weight%, carbon (C): 0.15 to 0.25%, manganese (Mn): 1.5 to 2.5%, silicon (Si): 0.1 to 1.0%, titanium (Ti): 0.01 to 0.05%, boron (B): 5 A hot rolling step of hot finishing and rolling the steel slab containing ˜30 ppm, the balance Fe and other unavoidable elements above an Ar ₃ temperature, a winding step of winding the rolled steel sheet at 500 to 800 ° C., and the wound steel sheet 50 to Cold rolling step that is cold rolled at 90% reduction rate, the cold rolled steel plate is maintained at an annealing temperature of 750 ~ 850 ℃ for more than 30 seconds in the continuous annealing line and then cooled to a temperature range of 250 ~ 450 ℃. After holding for 50 seconds or more, the annealing step of cooling, and the second step of rolling the annealing steel sheet treated as described above with a reduction ratio of 10% or less.

Further, it is preferable that the product of the C, Mn and B contents satisfies the relationship of 1.13 x 10 ^-4 <wt% C x wt% Mn x wt% B <1.875 x 10 ^-3 .

In addition, the hot rolling step is preferably performed so that the thickness of the hot rolled steel sheet is 1.0 ~ 3.0mm.

In addition, the annealing cooling is preferably performed at a cooling rate of 10 to 50 ° C / sec, more preferably 10 to 30 ° C / sec.

In addition, the moving speed of the steel sheet during the annealing is preferably 100 ~ 500m / min.

According to the present invention it is possible to produce a high-strength cold-rolled steel sheet without maintaining the excellent formability and very small secondary rolling is less than 10% in the rolling direction formability. In addition, there is an effect applicable to the continuous annealing to produce a general product in the production of cold rolled steel sheet having the above characteristics.

1 is an optical tissue picture of the invention steel according to the present invention and comparative steel B outside the scope of the present invention, (a) shows a tissue picture of the invention steel made of invention steel B, (b) is a comparative steel B Represents a tissue picture of.

The present invention is to make a high-strength ultra-thin steel sheet maintaining excellent moldability, characterized in that it comprises a bainite structure in order to maintain excellent moldability, the carbon content of 0.15 ~ 0.25% carbon to secure strength Steel plate is used.

In addition, in order to obtain low temperature transformation structure even at low cooling rate, high hardenability is obtained by controlling content of Mn and B in relatively inexpensive steel, excluding expensive alloy elements such as Nb and Mo, which are generally added in steel. It is possible to form a low-temperature transformation structure during annealing at a rate of 30 ° C./sec or less, which is a cooling rate during annealing in a continuous annealing furnace (CAL), thereby making it applicable to continuous annealing. In addition, even if secondary rolling is not performed, the secondary steel is subjected to secondary rolling in a range in which the rolling direction formability does not deteriorate compared to the high strength ultra-thin material for molding using the secondary rolling which is generally used. In addition, it is characterized in that it is possible to obtain a high strength ultra-thin material for molding having a higher hardness while maintaining the same level of moldability.

Hereinafter, the present invention will be described in detail.

First, the steel composition of this invention is demonstrated. The content of each element below represents weight percent.

It is preferable to make content of carbon (C) into 0.15 to 0.25%.

The C is preferably contained 0.15% or more in order to control the structure for securing sufficient strength in the production of ultra-thin cold rolled steel sheet. However, if the content of C exceeds 0.25%, problems may occur such as carbide precipitation, workability of the steel sheet, the possibility of cold rolling, deterioration of the shape, and the flowability during annealing, so the content of C is limited to 0.15 to 0.25%. It is desirable to.

The content of manganese (Mn) is preferably 1.5 to 2.5%.

The Mn lowers the Ar ₃ temperature and improves its hardenability upon cooling, thereby delaying the formation of transformation phases such as pearlite when cooling at a low cooling rate, thereby allowing the bainite phase to be formed at a general cooling rate. . Moreover, it is also an essential component added in order to prevent the red light brittleness of the impurity S. In order to exhibit such an effect, it is preferable to add 1.5% or more, but if it exceeds 2.5%, problems may occur in cold rolling property, brittleness of the slab, etc., so the content of Mn is preferably limited to 1.5 to 2.5%. The Ar ₃ temperature is an inverse transformation temperature for forming an austenite pool for causing transformation during cooling of the continuous annealing process.

The content of silicon (Si) is preferably 0.1 to 1.0%.

Si is an element that plays a role of deoxidizer and solid solution strengthening, and it is preferable to add 0.1% or more in order to exhibit such an effect, but if it exceeds 1.0%, crack brittleness problem occurs. It is preferable.

The content of titanium (Ti) is preferably set to 0.01 to 0.05%.

Ti serves as a scavenger for suppressing the formation of boron nitride formed by the bonding of N and B remaining in the steel as an element added to more reliably obtain the effect of B. Therefore, the content of Ti is determined in proportion to the content of N remaining in the steel, it is preferable to limit to 0.01 ~ 0.05%.

The content of boron (boron, B) is preferably limited to 5 ~ 30ppm.

B is a major element that improves the hardenability together with Mn so that the bainite phase can be formed even at a general cooling rate during annealing. It is preferable to add more than 5ppm to show this effect, but when it is added more than 30ppm, excessive grain boundary boron precipitates are formed, which adversely affects the properties of the steel, so the content of B is limited to 5 to 30ppm. desirable.

It is preferable that the product of the contents of C, Mn, and B satisfies the relationship of 1.13 × 10 ⁻⁴ <wt% C × wt% Mn × wt% B <1.875 × 10 ⁻³ . If the product of the content is less than 1.13 × 10 ⁻⁴ , the Ar ₃ temperature rises and the hardenability is low, and thus bainite is hardly formed. If the product of the content is more than 1.875 × 10 ⁻³ , the rolling property is poor and brittle. It is preferable to satisfy the above relationship because there is a possibility of occurrence.

In addition to the above components, P, Al, S, N and the like can be included. Al may be included up to 0.06%, and P, S, and N may be included up to 0.03%, respectively.

The microstructure of the cold rolled steel sheet of the present invention is bainite single phase or 70 vol.% Or more of bainite as a main phase, and is composed of residual ferrite. Since the bainite structure can be obtained at a general cooling rate, the fabrication is less distortion compared to the martensitic steel obtained by performing rapid cooling of 50 ° C / sec or more, thereby improving workability and formability.

In addition, the microstructure of the cold-rolled steel sheet of the present invention may contain up to 30 vol.% Of ferrite, and the ferrite serves to secure ductility of steel.

Hereinafter, the manufacturing conditions of the cold rolled steel sheet of the present invention will be described.

In the present invention, the steel slab formed as described above is heated, hot finished rolling at an Ar ₃ temperature or higher, and wound at 500 to 800 ° C.

The steel slab heating temperature is preferably limited to 1100 ℃ or more in order to ensure a stable hot rolling finish temperature.

In addition, the hot rolling finish temperature is preferably limited to the Ar ₃ temperature or more in order to roll in the austenitic single-phase region, and more preferably hot rolling finish temperature is Ar ₃ It is -950 degreeC.

The winding temperature is preferably limited to 500 ° C. or more in order to obtain cold rolling property, but when the temperature exceeds 800 ° C., grains may be coarsened, so the winding temperature is preferably limited to 500 ° C. to 800 ° C.

Although the thickness of the hot rolled steel sheet is not particularly limited, it is preferable that the thickness of the hot rolled steel sheet is reduced to a thickness of 1.0 to 3.0 mm in order to be manufactured into an ultra-thin cold rolled steel sheet.

In the present invention, a large amount of the precipitation-reinforced element was not added as described above, and the winding temperature was controlled to 500 ° C. or higher, so that no hard structure was formed during hot rolling. Therefore, the final strength of the hot rolled steel was not so high. The load can be reduced.

Next, after cold rolling the hot rolled hot rolled steel sheet at a reduction ratio of 50 to 90%, the cold rolled steel sheet is maintained at an annealing temperature of 750 to 850 ° C. for at least 30 seconds in a continuous annealing line, and then 250 to 450 It has a high strength and high formability by cooling at a cooling rate of 10 to 50 ° C./sec to a temperature section (overaging temperature section) of C, maintaining at least 50 seconds (overaging) at this temperature, and then performing continuous annealing to cool. Ultra-thin cold rolled steel sheet is produced.

The cold reduction rate is preferably limited to 50 to 90%. If the cold reduction ratio is less than 50%, it is difficult to secure a target thickness, and if it exceeds 90%, there is a problem of inferior rollability.

If the annealing temperature is less than 750 ℃, there is a problem that the reverse transformation to austenite does not occur sufficiently, and if it exceeds 850 ℃ heat buckle or the like easily occurs, the annealing temperature is 750 ~ 850 ℃ It is preferable to limit to.

If the holding time at the annealing temperature is less than 30 seconds, since reverse transformation to austenite does not sufficiently occur, the holding time is preferably maintained for 30 seconds or more.

Since bainite is not sufficiently formed when the cooling stop temperature (overaging temperature) is less than 250 ° C or more than 450 ° C, the cooling stop temperature (overaging temperature) is preferably limited to 250 to 450 ° C. In addition, pearlite may be formed when the cooling rate is less than 10 ° C./second, and martensite may be formed when it exceeds 50 ° C./second, and the cooling rate may be limited to 10 to 50 ° C./second. It is preferable. More preferable cooling rate is 10-30 degree-C / sec.

If the holding time (overaging time) is less than 50 seconds, bainite is not sufficiently formed, and therefore, the holding time (overaging time) is preferably limited to 50 seconds or more.

In the continuous annealing, when the moving speed of the steel sheet is less than 100 mm / min, pearlite may be formed, and when it exceeds 500 m / min, martensite may be formed. Thus, in order to generate a bainite phase, 100 It is preferable to limit to ˜500 m / min.

In the present invention, through an active ingredient control as described above, when the annealing occurs in the temperature range of 750 ~ 850 ℃ reverse transformation occurs in the austenite phase, the structure of the steel plate 250 to 450 in the state that is not transformed from the austenite phase to pearlite, etc. Cooling is carried out to a temperature range of 占 폚, and the bainite transformation occurs by maintaining this temperature for 50 seconds or more.

The cold rolled steel sheet manufactured as described above has a bainite single phase or bainite of 70 vol.% Or more as a main phase and includes residual ferrite.

In the cold rolled steel sheet, when the steel sheet is r = 0 L-bending (bending) forming test, it is preferable that the number of cracks that can be visually observed at the corner thereof is 2 or less per unit m.

The cold rolled steel sheet preferably has a thickness of 0.5 mm or less in order to be manufactured into an ultrathin steel sheet.

The cold rolled steel sheet includes secondary rolling after cold rolling-continuous annealing. In order to prevent the phenomenon that rolling direction moldability worsens, the rolling amount of secondary rolling is 10% or less of rolling reduction is preferable.

As described above, the present invention removes expensive elements such as Nb and Mo and promotes bainite transformation during continuous annealing without increasing initial strength by using alloys such as Mn and B which are relatively low cost. By using, by having a high hardness already before the secondary rolling, by performing the secondary rolling, it is characterized in that a high strength ultra-thin material for molding can be obtained while maintaining the same level of moldability.

In addition, the present invention is a low formability characteristic of the martensite structure of a similar level of strength compared to the prior art, such as performing a rapid cooling of 50 ℃ / sec or more to utilize the martensite structure, etc. to cause transformation in the low carbon series It can overcome the, and has the advantage of preventing distortion due to shear (shear) transformation.

In addition, the present invention lowers the cooling rate at the time of transformation in the continuous annealing process to a cooling rate of the general continuous annealing furnace (CAL) level to form a low-temperature high-strength transformation structure that can be applied to the general continuous annealing process without the addition of expensive alloys or fast cooling rate It has the advantage to be obtained.

Hereinafter, the present invention will be described in more detail with reference to Examples.

(Example 1)

The steel having the composition of Table 1 is hot rolled (heating temperature: 1250 ℃, finish rolling temperature: 900 ℃, hot rolled steel sheet thickness: 2.7mm and winding temperature: 600 ℃), and then cold rolled to the manufacturing conditions of Table 2 After cold rolling of primary cold rolling: 89%, thickness: 0.3mm), and then annealed to the manufacturing conditions of Table 3 below, yield strength and total elongation, hardness and formability (whether cracking occurs during L-bending) The yield strength and total elongation are shown in Table 2, hardness in Table 4, and moldability evaluation results (with or without cracks) in Table 5, respectively.

Moreover, the optical structure photograph of the invention steel and the comparative steel was observed, and the result is shown in FIG. Figure 1 (a) is a representative picture of the invention steel shows a structure picture of the invention steel B annealed at 800 ℃, (b) is a representative picture of the comparative steel structure of the comparative steel B subjected to 14% secondary rolling after annealing Represents a picture.

In the case of the invention steel, it was confirmed that it has a needle-like single-phase tissue, in the case of the comparative steel it was confirmed that the mixed abnormal tissue of pearlite and ferrite (black) represented.

Steel grade
Composition (wt%) C Mn Si P S Al Ti B N Comparative Steel A 0.18 0.76 0.012 0.016 0.0044 0.036 0.01 - 0.0034 Comparative Steel B 0.18 1.23 0.012 0.08 0.0049 0.021 - - 0.0042 Invention river 0.19 2.24 0.17 0.01 0.008 0.03 0.016 0.0016 0.0053

In Table 2, yield strength and elongation according to the secondary reduction ratio for comparative steels A and B

In the case of the invention steel, the yield strength and the elongation at the time of performing secondary rolling at the rolling reduction rates of 6% and 10% without performing secondary rolling immediately after continuous annealing are shown.

In Tables 2 and 5 below, tissue B represents bainite, F represents ferrite, and P represents pearlite.

Secondary rolling amount (%) 0 6 10 14 20 25 30 40 50 54 group Comparative Steel A Yield strength (MPa) 390 - - 499 - 516 - 636 706 730 F single phase % Total elongation 25 - - 6.41 - 3.22 - 1.54 1.88 3.1 Comparative Steel B Yield strength (MPa) 447 553 685 657 720 - 770 835 - - F + P % Total elongation 23.7 14.8 6.0 9.5 5.2 - 3.9 3.6 - - Invention steel
Condition A Yield strength (MPa) 650 700 800 - - - - - - - B + F (9 vol.%) % Total elongation 5.0 4.3 3.5 - - - - - - - Invention steel
Condition B Yield strength (MPa) 791 900 1000 - - - - - - - B phase
% Total elongation 6.23 5.5 4.2 - - - - - - -

Invention steel
Condition Heating rate
(° C / sec) Crack temperature
(℃) Crack time
(second) Cooling rate
(° C / sec) Aging temperature
(Cooling stop temperature)
(℃) Aging time
(Holding time) (sec) Condition A 7 750 97 15 350 217 Condition B 7 800 97 15 350 217

Hardness
(HV500g) Comparative Steel A Comparative Steel B Invention steel
Condition A Invention steel
Condition B Hardness
(HV500g) 220 212 400 470

Table 5 below shows the results of the test of the formability of the inventive steel and the comparative steel. In the L-bending test, crack formation is affected by die clearances, and thus the gap between dies is almost reduced. A poor condition of 0 was assumed and a 90 degree L-bending experiment was performed using r = 0 bending.

And the annealing condition of the invention steel is a low temperature annealing temperature of the invention steel made at 700 ℃ level does not sufficiently inverse transformation, so the bainite fraction in the tissue is small, the target high strength could not be obtained, so the test piece for forming test It experimented by limiting the annealing temperature to 750 degreeC, 780 degreeC, and 800 degreeC. In addition, after the annealing of the invention steel as described above, secondary rolling of 6% was performed, and the experiment was carried out twice. In Table 5, if x is used to indicate cracking, △ means no cracking occurs but necking occurs before the cracking occurs, and ○ means cleared surface without cracking ( clear surface).

Crack occurrence (○, △, ×) L-bending 180 degree folding group Cooling rate (℃ / sec) 10 15 20 30 10 15 20 30 Inventive Steel Annealing Temperature: 750 ℃ × × × × × × × × × × × × × × × × B + F Inventive Steel Annealing Temperature: 780 ℃ ○ ○ △ △ △ △ × × ○ ○ △ △ × × × × Inventive steel annealing temperature: 800 ℃ ○ ○ ○ △ △ △ × × ○ ○ ○ × × × × × Crack occurrence (○, △, ×) L-bending 180 degree folding group Secondary rolling amount (%) 10 20 30 40 10 20 30 40 Comparative Steel A ○ ○ ○ ○ × × × × ○ ○ × × × × × × F single phase Comparative Steel B ○ ○ ○ ○ △ △ × × ○ ○ △ △ × × × × F + P

On the other hand, in the case of the actual ultrathin material, due to the error of the yield strength due to the ultrathin, in addition to the yield strength, a lot of hardness is used as a measure of strength.

As shown in Table 4, when comparing the invention steel and the comparative steel, it can be seen that the difference in hardness value is significantly higher than the difference in yield strength value. This phenomenon is generally inferred from the fact that the hardness value is proportional to the tensile strength rather than the yield strength of the steel.In the case of the inventive steel, the secondary steel is compared with the comparative steels A and B, which have some degree of work hardening through secondary rolling. Rolling can be performed at a level of 10% or less to maintain the elongation of the structure itself, and the base structure itself is characterized by high yield strength value due to the high bainite structure.

As shown in Table 5, when the invention steel is cooled at a low cooling rate of less than 10 ℃ / second at an annealing temperature of 780 ℃ or more, the necking or cracking occurs in the specimen even during L- bending or even worse folding test I could confirm that I did not. On the contrary, in the case of comparative steel A, the yield strength of the comparative steel A which was lower than the yield strength when the secondary rolling of 6% of the invention steel A showing the lowest yield strength value among the inventive steels of the experimental example was performed. Even in the case of the test piece which performed the secondary rolling of 40%, all the cracks formed in the test piece after secondary rolling and bending, and it broke. In addition, when comparing the inventive steel B and the comparative steel B, the comparative steel B not only has a thermal yield strength compared to the inventive steel B, but also cracks the test specimens in all cases in which secondary rolling of 30% or more having a relatively high yield strength is applied. This occurred or necking occurred.

As can be seen in this experimental example, in the case of the steel of the present invention, the annealing condition is a continuous annealing to produce a general product, there is an advantage that can produce a high strength steel of 1000MPa class.

Claims (10)

delete delete delete delete delete By weight%, carbon (C): 0.15 to 0.25%, manganese (Mn): 1.5 to 2.5%, silicon (Si): 0.1 to 1.0%, titanium (Ti): 0.01 to 0.05%, boron (B): 5 A hot rolling step of hot finishing rolling the steel slab containing ˜30 ppm, the balance Fe and other unavoidable elements at an Ar 3 temperature or more and 950 ° C. or less;
Winding step of winding the rolled steel sheet at 500 ~ 800 ℃;
A cold rolling step of cold rolling the wound steel sheet at a reduction ratio of 50 to 90%;
The cold rolled steel sheet having a thickness of 0.5 mm or less is maintained at an annealing temperature of 750 to 850 ° C. for at least 30 seconds in a continuous annealing line, and then cooled to a temperature range of 250 to 450 ° C. and maintained at this temperature for at least 50 seconds, and then cooled. Annealing step; And a second rolling of the annealing steel sheet subjected to the annealing as described above at a rolling reduction of 10% or less, wherein the cooling rate is 10 to 30 ° C / sec during the annealing step, and the number of cracks in the corner portion is 2 per m. The method of manufacturing a high strength ultra-thin cold rolled steel sheet using cold rolling, which is less than or equal to and the moving speed of the steel sheet is 100 to 500 m / min.
The method of claim 6,
The method of producing a high strength ultra-thin cold rolled steel sheet using cold rolling, wherein the product of C, Mn, and B content satisfies a relationship of 1.13 × 10 ⁻⁴ <wt% C × wt% Mn × wt% B <1.875 × 10 ^{− 3} .
The method according to claim 6 or 7,
The hot rolling step is a method of manufacturing a high strength ultra-thin cold rolled steel sheet using cold rolling to be performed so that the thickness of the hot rolled steel sheet is 1.0 ~ 3.0mm.
The method according to claim 6 or 7,
The method of manufacturing a high strength ultra-thin cold rolled steel sheet using cold rolling at the time of annealing, the cooling rate is 10 ~ 30 ℃ / sec.
delete

Images